Two-stage injection molding machines are widely used in accordance with the requirements of high precision and good quality in injection molding. A typical two-stage injection molding machine is illustrated in FIG. 5, which comprises a feeding portion 50 and an injection portion 55, wherein the feeding portion 50 provides metering and check functions to achieve a precise metering effect and thereby improve the quality of injection molding.
As shown in FIG. 5, the two-stage injection molding machine is operated by allowing a motor 56 to drive a metering screw 57 to rotate, such that plastic material in a material hopper 58 enters a tube 59 to mix and melt to form melted plastics, and a check shaft 61 in a check shaft seat 60 is opened to allow the melted plastics to enter an injection tube 65. After a required injection volume and a back pressure of the melted plastics are provided, the check shaft 61 is closed. Then, a driver 66 drives an injection rod 67 to move forward and force the melted plastics to be injected.
As shown in FIGS. 6A and 6B, the check shaft 61 is used to control the opening or closing of a runner 62 of the check shaft seat 60 by rotation of the check shaft 61, so as to prevent the melted plastics of high pressure in the injection tube 65 from reversely flowing into the tube 59 to cause an unstable injection pressure and unstable measurement of the melted plastics. Such design is very important to precise injection molding having a strict requirement of injection precision. U.S. Pat. No. 5,858,420 discloses a two-stage injection molding machine as shown in FIG. 7, which employs a similar design having axial movement of a check shaft 71 cooperate with a check shaft seat 70 to achieve the metering and check functions.
However, for the foregoing two-stage injection molding machines shown in FIGS. 5 to 7, during practical operation thereof, relative motion between the check shaft 61, 71 and the corresponding check shaft seat 60, 70 causes a gap therebetween and easily leads to overflow and leakage of the melted plastics. In order to solve this problem, conventionally, the gap between the check shaft 61, 71 and the check shaft seat 60, 70 is made smaller (i.e. tolerance is decreased), or an anti-leakage device is additionally provided to prevent the overflow situation. Since the melted plastics of high pressure easily flow into the gap during the injection and metering processes, if the gap is made too small, the check shaft 61, 71 may be locked and fail to rotate. Furthermore, for plastic material added with fibers or powders, if it flows into the gap and cannot be discharged, it may be burned and broken due to overheating; if such residues of the plastic material is mixed with the melted plastics, the quality of finished products would be seriously degraded (especially for transparent finished products of injection molding).
Therefore, the problem to be solved here is to provide a discharge apparatus applied in a two-stage injection molding machine, to prevent locking of a check shaft and pollution from degraded residues of plastic material.